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arXiv:2410.10642 · Physical Review A

Cooling a strongly interacting quantum gas by interaction modulation

D. Eberz, A. Kell, Moritz Breyer, Michael Köhl

Published Jul 15, 2025• 1 citation
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Abstract

We present a cooling method for a strongly-interacting trapped quantum gas. By applying a magnetic field modulation with frequencies close to the binding energy of a molecular bound state we selectively remove dimers with high kinetic energy from the sample. We demonstrate cooling of the sample over a wide range of interaction strengths and measure a high cooling efficiency of $γ=4$ that exceeds all previous cooling near Feshbach resonances.

DOI
10.1103/rvt9-hkll

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We present a cooling method for a strongly-interacting trapped quantum gas. By applying a magnetic field modulation with frequencies close to the binding energy of a molecular bound state we selectively remove dimers with high kinetic energy from the sample. We demonstrate cooling of the sample over a wide range of interaction strengths and measure a high cooling efficiency of γ=4γ=4γ=4 that exceeds all previous cooling near Feshbach resonances.
A method is proposed that utilizes a Feshbach resonance to cool an atomic gas to temperatures of quantum degeneracy. Through application of a sequence of alternating adiabatic and nonadiabatic magnetic field ramps in the region of a Feshbach resonance, a large number of atom pairs in a trapped gas lose energy at the expense of a relatively small number of pairs which gain energy. After removal from the trap of the atom pairs which gain energy, the remaining reduced energy atoms will equilibrate to a lower temperature.
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We study the process of associating molecules from atomic gases using a magnetic field modulation that is resonant with the molecular binding energy. We show that maximal conversion is obtained by optimizing the amplitude and frequency of the modulation for the particular temperature and density of the gas. For small modulation amplitudes, resonant coupling of an unbound atom pair to a molecule occurs at a modulation frequency corresponding to the sum of the molecular binding energy and the relative kinetic energy of the atom pair. An atom pair with an off-resonant energy has a probability of association which oscillates with a frequency and time-varying amplitude which are primarily dependent on its detuning. Increasing the amplitude of the modulation tends to result in less energetic atom pairs being resonantly coupled to the molecular state and also alters the dynamics of the transfer from continuum states with off-resonant energies. This leads to maxima and minima in the total conversion from the gas as a function of the modulation amplitude. Increasing the temperature of the gas leads to an increase in the modulation frequency providing the best fit to the thermal distribution, and weakens the resonant frequency dependence of the conversion. Mean-field effects can alter the optimal modulation frequency and lead to the excitation of higher modes. Our simulations predict that resonant association can be effective for binding energies of order $h\ifmmode\times\else\texttimes\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$.
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We use laser light near resonant with a molecular bound-to-bound transition to control a magnetic Feshbach resonance in ultracold Fermi gases of 40{}^{40}40K atoms. The spectrum of excited molecular states is measured by applying a laser field that couples the ground Feshbach molecular state to electronically excited molecular states. Nine strong bound-to-bound resonances are observed below the 2P1/2+2S1/2{}^{2}{P}_{1/2}+{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}2P1/2​+2S1/2​ threshold. We use radio-frequency spectroscopy to characterize the laser-dressed bound state near a specific bound-to-bound resonance and show clearly the shift of the magnetic Feshbach resonance using light. The demonstrated technology could be used to modify interatomic interactions with high spatial and temporal resolutions in the crossover regime from a Bose-Einstein condensate to a Bardeen-Cooper-Schrieffer superfluid.